1. Field of the Invention
The present invention relates to a semiconductor laser device having an end-facet window structure, and a process for producing such a semiconductor laser device.
2. Description of the Related Art
In conventional semiconductor laser devices, when optical output power is increased, currents generated by optical absorption in vicinities of end facets generate heat, i.e., raise the temperature at the end facets. In addition, the raised temperature reduces the semiconductor bandgaps at the end facets, and therefore the optical absorption is further enhanced. That is, a vicious cycle is formed, and the end facets are damaged. This damage is the so-called catastrophic optical mirror damage (COMD). Thus, the maximum optical output power is limited due to the COMD. In addition, when the optical power reaches the COMD level, the optical output deteriorates with time. Further, the semiconductor laser device is likely to suddenly break down due to the COMD. It is known that high reliability in high output power operation can be achieved when window structures are formed in the vicinities of end facets, i.e., crystals having a greater bandgap than an active layer are formed in the vicinities of the end facets, so as to prevent the light absorption in the vicinities of end facets.
For example, Kazushige Kawasaki et al. (xe2x80x9c0.98 xcexcm band ridge-type window structure semiconductor laser (1),xe2x80x9d Digest 29a-PA-19, 1997 Spring JSAP Annual Meeting, The Japan Society of Applied Physics) disclose a semiconductor laser device in the 980 nm band, which has a window structure formed by injecting Si ions into end regions of a ridge structure and disordering an In0.2Ga0.8As quantum well by thermal diffusion. However, the process for producing the above semiconductor laser device is very complicated and long since the vicinities of end facets are required to be insulated by injection of H ions after the injection of the Si ions in the vicinity of the active layer in order to prevent a current flow in the vicinities of end facets.
In addition, when the active layer contains aluminum, the reliability of the semiconductor laser device is decreased due to oxidation of aluminum. In particular, when a window structure is formed by removing near-edge portions of the active layer and regrowing semiconductor layers in the near-edge portions, aluminum is exposed on the regrowth boundary. Therefore, the reliability of the semiconductor laser device is further decreased.
An object of the present invention is to provide a semiconductor laser device which does not contain aluminum in an active layer, has a window structure being non-absorbent of light in vicinities of end facets, and is reliable in a wide output power range from low to high output power.
Another object of the present invention is to provide a process which can produce, by a simple process, a semiconductor laser device which does not contain aluminum in an active layer, has a window structure being non-absorbent of light in vicinities of end facets, and is reliable in a wide output power range from low to high output power.
(1) According to the first aspect of the present invention, there is provided a semiconductor laser device comprising: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a first lower optical waveguide layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 having a first bandgap, and formed on the lower cladding layer, where 0xe2x89xa6x2xe2x89xa60.3 and x2=0.49y2; an intermediate layer made of Inx5Ga1xe2x88x92x5P and formed on the first lower optical waveguide layer, where 0 less than x5 less than 1; a second lower optical waveguide layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 having the first bandgap, and formed on the intermediate layer except for near-edge areas of the first intermediate layer located in first vicinities of opposite end facets of the semiconductor laser device so as to leave first portions of spaces in the first vicinities of opposite end facets, where 0xe2x89xa6x2xe2x89xa60.3 and x2=0.49y2; a compressive strain active layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 having a second bandgap smaller than the first bandgap, and formed on the second lower optical waveguide layer so as to leave second portions of the spaces in second vicinities of opposite end facets, where 0 less than x1xe2x89xa60.4 and 0xe2x89xa6y1xe2x89xa60.1; an upper optical waveguide layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 having the first bandgap, and formed on the compressive strain active layer so as to leave third portions of the spaces in third vicinities of opposite end facets, where 0xe2x89xa6x2xe2x89xa60.3 and x2=0.49y2; a cap layer made of Inx5Ga1xe2x88x92x5P and formed on the upper optical waveguide layer so as to leave fourth portions of the spaces in fourth vicinities of opposite end facets, where 0 less than x5 less than 1; a non-absorbent layer made of Inx6Ga1xe2x88x92x6As1xe2x88x92y6Py6 having a third bandgap greater than the second bandgap, and formed over the cap layer so that the spaces are filled with the non-absorbent layer, where 0xe2x89xa6x6xe2x89xa60.3 and x6=0.49y6; an upper cladding layer of a second conductive type, formed on the non-absorbent layer; and a contact layer of the second conductive type, formed on the upper cladding layer.
In addition, each of the lower and upper cladding layers, the first and second lower optical waveguide layers, the upper optical waveguide layer, and the non-absorbent layer are assumed to have such a composition as to lattice-match with the active layer.
In this specification, the lattice matching is defined as follows.
When c1 and c2 are lattice constants of first and second layers, respectively, and the absolute value of the amount (c1xe2x88x92c2)/c2 is equal to or smaller than 0.001, the first layer is lattice-matched with the second layer. For example, when cs and c are the lattice constants of a substrate and a layer grown above the substrate, respectively, and the absolute value of the amount (cxe2x88x92cs)/cs is equal to or smaller than 0.001, the layer grown above the substrate is lattice-matched with the substrate.
Further, the first conductive type is different in polarity of carriers from the second conductive type. That is, when the first conductive type is p type, and the second conductive type is n type.
Preferably, the semiconductor laser device according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (v).
(i) The contact layer may be formed on the upper cladding layer except for near-edge areas of the upper cladding layer located in fifth vicinities of the end facets of the semiconductor laser device, and an insulation layer may be formed on the near-edge areas of the upper cladding layer so as to prevent current injection through the near-edge areas of the upper cladding layer.
(ii) Each of the lower and upper cladding layers may be made of one of Alz1Ga1xe2x88x92z1As and Inx3(Alz3Ga1xe2x88x92z3)131 x3As1xe2x88x92y3Py3, where 0.2xe2x89xa6z1xe2x89xa60.8, x3=0.49y3, 0.9 less than y3xe2x89xa61, and 0xe2x89xa6z3xe2x89xa61.
(iii) Regions of the semiconductor laser device above at least a mid-thickness of the upper cladding layer except for a stripe region of the semiconductor laser device may be removed so as to form a ridge and realize index guidance of light.
(iv) The semiconductor laser device according to the first aspect of the present invention may further comprise a current confinement layer made of one of Alz2Ga1xe2x88x92x2AS and In0.49Ga0.51P which lattice-match with GaAs, and formed above the upper optical waveguide layer so as to form an internal current confinement structure realizing index guidance of light, where 0.2 less than z2 less than 1.
(v) In order to compensate for the strain of the active layer, two Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 tensile strain barrier layers (0xe2x89xa6x4 less than 0.49y4, 0 less than y4xe2x89xa61) may be formed in vicinities of the active layer.
The strain xcex94a of the active layer and the strain xcex94b of the Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 tensile strain barrier layers can be expressed by
xcex94a=(caxe2x88x92cs)/cs,
and
xe2x80x83xcex94b=(cbxe2x88x92cs)/cs
where ca, cb, and cs are lattice constants of the active layer, the Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 tensile strain barrier layers, and the GaAs substrate, respectively.
In this case, in order to prevent damage to the crystals in the active region, it is preferable that the active layer and the tensile strain barrier layers satisfy the following inequalities,
xe2x88x920.25 nmxe2x89xa6xcex94axc3x97da+2xcex94bxc3x97dbxe2x89xa60.25 nm,
where da and db are respectively the thicknesses of the active layer and each of the tensile strain barrier layers.
(2) According to the second aspect of the present invention, there is provided a process for producing a semiconductor light emitting device, comprising the steps of: (a) forming above a GaAs substrate of a first conductive type a lower cladding layer of the first conductive type; (b) forming above the lower cladding layer a first lower optical waveguide layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2, where 0xe2x89xa6x2xe2x89xa60.3 and x2=0.49y2; (c) forming above the first lower optical waveguide layer an intermediate layer made of Inx5Ga1xe2x88x92x5P, where 0 less than x5 less than 1; (d) forming above the intermediate layer a second lower optical waveguide layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2, where 0xe2x89xa6x2xe2x89xa60.3 and x2=0.49y2; (e) forming above the second lower optical waveguide layer a compressive strain active layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1, where 0 less than x1xe2x89xa60.4 and 0xe2x89xa6y1xe2x89xa60.1; (f) forming above the compressive strain active layer a first upper optical waveguide layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 where 0xe2x89xa6x2xe2x89xa60.3 and x2=0.49y2; (g) forming above the first upper optical waveguide layer a first cap layer made of Inx5Ga1xe2x88x92x5P, where 0 less than x5 less than 1; (h) etching off near-edge portions of the first cap layer located in first vicinities of two opposite end facets of the semiconductor laser device with a hydrochloric acid etchant so as to produce first portions of spaces in the first vicinities of the two opposite end to facets of the semiconductor laser device; (i) etching off near-edge portions of the upper optical waveguide layer, the compressive strain active layer, and the second lower optical waveguide layer, located in second vicinities of the two opposite end facets of the semiconductor laser device, by using a sulfuric acid etchant and the first cap layer as a mask, so as to produce second portions of the spaces in second vicinities of two opposite end facets of the semiconductor laser device; (j) forming a second upper optical waveguide layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 over the first cap layer so that the spaces are filled with the second upper optical waveguide layer, where 0xe2x89xa6x2xe2x89xa60.3 and x2=0.49y2; (k) forming an upper cladding layer of a second conductive type over the second upper optical waveguide layer; and (1) forming a GaAs contact layer of the second conductive type above the upper cladding layer.
In addition, each of the lower and upper cladding layers, the first and second lower optical waveguide layers, the upper optical waveguide layer, and the non-absorbent layer are assumed to have such a composition as to lattice-match with the active layer.
The process according to the second aspect of the present invention may further comprise, between the steps (g) and (h), the steps of (m) forming a second cap layer made of GaAs, on the first cap layer; and (n) etching off near-edge portions of the second cap layer located in third vicinities of the two opposite end facets of the semiconductor laser device by using a sulfuric acid etchant so as to produce additional portions of the spaces. In this case, in the step (h), the second cap layer is used as a mask; and in the step (i), the second cap layer is etched off concurrently with the near-edge portions of the upper optical waveguide layer, the compressive strain active layer, and the second lower optical waveguide layer.
(3) The present invention has the following advantages.
(a) According to the present invention, the compressive strain active layer is made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1, the first and second lower optical waveguide layers and the upper optical waveguide layer are made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2py2, and the intermediate layer made of Inx5Ga1xe2x88x92x5P is formed between the first and second lower optical waveguide layers, where 0 less than x1xe2x89xa60.4, 0xe2x89xa6y1xe2x89xa60.1, 0xe2x89xa6x2xe2x89xa60.3 and x2=0.49y2. Therefore, when the upper optical waveguide layer, the active layer, and the second lower optical waveguide layer are etched with a sulfuric acid etchant, the etching automatically stops at the upper surface of the Inx5Ga1xe2x88x92x5P intermediate layer. Thus, the depth of the etching with the sulfuric acid etchant can be easily controlled, and the spaces in the vicinities of the end facets can be accurately produced by a simple process for realizing the end-facet window structure.
In addition, the end-facet window structure is realized by filling the accurately produced spaces with the Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 non-absorbent layer having a higher bandgap than the active layer. Therefore, it is possible to produce a reliable semiconductor laser device.
(b) The Inx5Ga1xe2x88x92x5P cap layer is formed above the upper optical waveguide layer. Therefore, the regrowth of the InGaAsP layer after the production of the spaces in the vicinities of the end facets becomes easy when the intermediate-stage structure after the production of the spaces in the vicinities of the end facets is placed in a phosphorus atmosphere.
In addition, an InGaAsP layer having a greater bandgap than that of the active layer is embedded in the vicinities of the end facets. That is, window structures which are non-absorbent of the oscillation light can be stably formed in the vicinities of the end facets. Therefore, it is possible to achieve high reliability in a wide output power range from low to high output power.
Further, since the depth of the spaces produced by the etching in the vicinities of the end facets is small, the upper surface of the regrown Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 non-absorbent layer easily becomes flattened, and therefore the index-guided structure can be easily formed through the entire length of the semiconductor laser device.
(c) The near-edge portions of the active layer are removed, and the spaces produced by the removal are filled with the non-absorbent layer by the regrowth. Therefore, it is possible to prevent current generation by light absorption in the vicinities of the end facets, and reduce heat generation in the vicinities of the end facets. Thus, the COMD level is greatly raised. That is, the semiconductor laser device according to the present invention is reliable even in operation with high output power.
In addition, when the semiconductor laser device has the aforementioned additional feature (i), the contact layer is not formed on the near-edge areas of the upper cladding layer. Therefore, the current for driving the semiconductor laser device is not injected into the near-edge portions of the semiconductor laser device. Thus, the heat generation in the vicinities of the end facets can be further reduced.
(d) The InGaP layer, which has a greater bandgap than InGaAsP, is formed between the first and second lower optical waveguide layers. Therefore, it is possible to prevent leakage of carriers from the active layer. Thus, the threshold current can be further lowered.
(e) Since the active layer does not contain aluminum, aluminum is not exposed on the regrowth boundary. Therefore, degradation caused by oxidation can be prevented. Thus, it is possible to produce a highly reliable semiconductor laser device.